1) Field of the Invention
The invention relates to a process and a purifier for the purification of hydrogen without provoking the production of methane and more particularly to the production of purified hydrogen with less than 20 parts per billion (parts per 10.sub.9 by volume) of methane.
2) Description of the Prior Art
As the semiconductor industry is developing integrated circuits with ever more increasing line densities it is required that the materials used in the manufacturing processes involved be of ever increasing purity. As hydrogen is one of the gases used in these processes it is therefore necessary to ensure that its impurity content be kept as low as possible. The main impurity gases in commercial hydrogen are nitrogen, moisture (water vapour), carbon monoxide and carbon dioxide in decreasing order of magnitude.
One method of purification of hydrogen has been to employ the known fact of the selective diffusion of hydrogen through palladium or palladium alloys. The rate of diffusion increases with the pressure difference between the opposite sides of a palladium barrier. Furthermore the operating temperature required for an economical throughput of purified hydrogen is 500.degree. C. or above. In addition, as the impurities contained within the hydrogen are blocked by the palladium barrier some means of removal must be provided. Takashi Eguchi et Al. in U.S. Pat. No. 3,368,329 provide one such apparatus. Another form of hydrogen purification by means of diffusion membranes is shown in Takashi Eguchi et al. in U.S. Pat. No. 3,534,531.
Such diffusion barriers while very efficient have several disadvantages. If the barrier is sufficiently thin to ensure a high throughput of purified hydrogen it becomes subject to mechanical failure with the undesirable leakage of impure hydrogen into the purified gas. The disadvantage is aggravated by the already high pressure difference between the two sides of the barrier. If the barrier thickness is increased to avoid mechanical failure then excessively high temperatures must be adopted to ensure a high throughput of purified gas. The use of high temperatures in the presence of hydrogen is also very dangerous due to the potential existence of explosive hydrogen-oxygen (air) mixtures whenever temperatures of 570.degree. C. or above are encountered. An increased barrier thickness also implies the use of more of the costly material palladium.
The use of various gas sorption materials in the purification of a wide variety of gases is also well known in the art. See for example United Kingdom patent application No. 2,177,079 A and No. 2,177,080 A wherein the purification of argon and nitrogen respectively, using two stages of purification, is discussed.
In the article "Removal of simple hydrocarbons from a rare gas by a 70% Zr-25% V-5% Fe getter" by M. A. George, J. H. Kiefer and J. P. Hessler published in Gas Separation and Purification, 1989, Vol. 3, pp. 50-55 there is described a two zone purifier which effectively removes methane from argon to less than 20 ppb. However this is for the purification of rare gases and not hydrogen.
An article with the title "Removal of nitrogen and methane from hydrogen by metal getters" by H. Helmbach, H. R. Ihle and C. H. Wu published in the Proceedings of the 13th Symposium on Fusion Technology (SOFT), Varese, Sep. 24-28, 1984, pp. 421-426 describes the removal of methane from hydrogen but found that "a measurable depletion of CH.sub.4 required temperatures in excess of 500.degree. C." and an "Appreciable depletion of OH.sub.4 occurs at about 600.degree. C." (See Abstract) when using Zr.sub.3 Al.sub.2 or Zr(V.sub.0.83 Fe.sub.0.17).sub.2 as getter materials for the removal of impurities. But there is no indication of the problems which arise when there is the simultaneous removal of carbon monoxide or carbon dioxide.
In another article "Application of SAES And HWT gas purifiers for the removal of impurities from helium-hydrogen gas mixtures" by H. Albrecht, U. Kuhnes and W. Asel published in the Journal of Less-Common Metals, Vol. 172-174 (1991) pp. 1157-1167 there is described the effect of the simultaneous sorption of various impurities. At page 1165 it states "For CH.sub.4 the getter temperature of 200.degree. C. was obviously too low to cause any measurable sorption effect. At 300.degree. C., however a surprising effect was found: an increase in concentration rather than the expected decrease. This can be explained by the formation of additional methane caused by the interaction of carbon monoxide and hydrogen during the passage through the getter". This effect is shown in FIG. 7 of the publication. It goes on to propose the use of two getter beds. "The first bed is operated in the range 200.degree.-250.degree. C. to reduce the concentration of CO and H.sub.2 and the second at a temperature of at least 400.degree. C. to remove effectively all the CH.sub.4 and N.sub.2. However when hydrogen is to be purified the hydrogen cannot be removed. As an alternative it proposes, in the conclusions to provide separation of hydrogen isotopes in molecular form by using a Pd-Ag diffused in a first purification step. This reintroduces all the disadvantages of the palladium diffusors previously discussed.